I have got a very irritating problem at my PC when I try to open my hotmail and yahoo accounts. I do log in as always, typing username and password. The page opens for 5 seconds (or less) and then it changes to a completely different page.

I already tried both Ad-aware and spybot. They helped at the first time but have no effect anymore.

Thanks for showing interest in my problems. It's starting to make me mad.

Actually it does not switch to another url. I did a "print screen" of the screen that appears after the failed log-in. It's not very clear (had to do some rearrangements in Paint) but I think you can get the idea.

The best solution would be reinstalling everything, but I have no CD with an operating system at hand.

This is clearly a browser hijacker.
Many of them are difficult to remove if you are not familiar with what happens.
Here’s an attempt:
1)Disable system restore.
2)Manually remove all temporary internet files, cookies and empty the temp directories of all users.
You might need to enable the ‘Show hidden and system files”
3)Scan for viruses with NAV and AVG: http://free.grisoft.com/freeweb.php/doc/2/
4)Download Hijackthis: http://www.spychecker.com/program/hijackthis.html
5)Disconnect from the internet
6)Run Hijackthis and post the log.